It is apparent that in future battery systems will be used increasingly both in stationary applications and in vehicles such as hybrid vehicles and electric vehicles. In order to be able to meet the requirements which are made for the respective application in terms of voltage and available power, a large number of battery cells are connected in series. Since the current which is made available by such a battery must flow through all the battery cells, and a battery cell can only conduct a limited current, battery cell are often additionally connected in parallel in order to increase the maximum current.
FIG. 1 shows a customary battery 10 in a detailed block diagram. A plurality of battery cells 11 are connected in series and optionally additionally in parallel in order to obtain a high output voltage (series circuit) and battery capacity (parallel circuit) which is desired for a respective application. A charging and disconnecting device 14 is connected between the positive pole of the battery cells and a positive battery terminal 12. It is optionally also possible to connect a disconnecting device 15 between the negative pole of the battery cells and a negative battery terminal 13. The disconnecting and charging device 14 and the disconnecting device 15 each comprise a contactor 16 or 17, which is provided for disconnecting the battery cells 11 from the battery terminals 12, 13, in order to connect the battery terminals 12, 13 in a voltage-free fashion. Owing to the high direct voltage of the series-connected battery cells 11 there is otherwise a considerable potential hazard for maintenance personnel or the like. In addition, a charging contactor 18 with a charging resistor 19 which is connected in series with the charging contactor 18 is provided in the charging and disconnecting device 14. The charging resistor 19 limits a charging current for the buffer capacitor connected into the direct voltage intermediate circuit of a customary battery-fed drive system, if the battery is connected to the direct voltage intermediate circuit. For this purpose, the contactor 16 is firstly left open, and only the charging contactor 18 is closed, with the result that a current which is limited by the charging resistor 19 and which can reach a maximum current equal to the battery voltage divided by the resistance of the charging resistor 19. If the voltage at the positive battery terminal 12 at least approximately reaches the battery voltage, the contactor 16 can be closed and, if appropriate, the charging contactor 18 can be opened. The contactors 16, 17 and the charging contactor 18 considerably increase the costs for a battery 10 since stringent requirements are made of their reliability and the currents which they are to conduct.
The series connection of a high number of battery cells means that, in addition to the high overall voltage, there is the problem that the entire battery fails if a single battery cell fails because the battery current has to be able to flow in all the battery cells owing to the series connection. Such a failure of the battery can lead to a failure of the entire system. In the case of an electric vehicle, a failure of the drive battery causes the vehicle to become immobile, and in other devices, such as for example the rotor blade adjustment in the case of wind turbine plants this can even lead to dangerous situations in the case of strong wind. For this reason, a high level of reliability of the battery is advantageous. According to the definition, the term “reliability” means the ability of a system to operate correctly for a predefined time.
A widespread approach for increasing the service life of a battery is what is referred to as cell balancing. It is based on the idea of loading all the battery cells of a battery as uniformly as possible so that an individual battery cell is not discharged prematurely causing the entire battery to fail even though sufficient electrical energy is still available in the other battery cells. A prematurely discharged battery cell also constitutes a considerable risk for safe operation because the discharged battery cell starts, from the point of view of the remaining battery cells, to constitute a load which can heat up greatly owing to the current which continues to flow. If a battery continues to be operated even though a battery cell is already discharged, there is the risk of destruction of the battery cell and therefore of continuous failure of the entire battery. The prior art therefore contains various approaches to cell balancing which locate a battery cell with a relatively high cell voltage and selectively discharge it with respect to the other battery cells (resistive cell balancing). Since in the case of resistive cell balancing the energy which is extracted by the cell balancing is lost, inductive cell balancing was also proposed in which the electrical energy which is extracted from the battery cell to be discharged is fed to another battery cell. However, in this context there are also electrical power losses and in addition there has to be a high degree of expenditure on circuitry by using coils which are costly and voluminous.